Causes

Although the causative agent, P. carinii, is often classified as a protozoa, its structure and function are closer to a fungus. Human transmission occurs by the airborne route, where it multiplies aggressively in the alveoli. Incubation is estimated to be between 4 and 8 weeks. Individuals at risk for PCP are immunocompromised and have conditions such as HIV infection or are premature infants. Other vulnerable populations include patients who receive corticosteroid therapy and organ transplantation.